The present invention relates to an electrochemical cell, and especially relates to an electrochemical cell capable of operating at high differential pressure.
Electrochemical cells are energy conversion devices, usually classified as either electrolysis cells or fuel cells. A proton exchange membrane electrolysis cell functions as a hydrogen generator by electrolytically decomposing water to produce hydrogen and oxygen gases, and functions as a fuel cell by electrochemically reacting hydrogen with oxygen to generate electricity.
Referring to FIG. 1, a partial section of a typical fuel cell 10 is detailed. In fuel cell 10, hydrogen gas 12 and reactant water 14 are introduced to a hydrogen electrode (anode) 16, while oxygen gas 18 is introduced to an oxygen electrode (cathode) 20. The hydrogen gas 12 for fuel cell operation can originate from a pure hydrogen source, methanol or other hydrogen source. Hydrogen gas electrochemically reacts at anode 16 to produce hydrogen ions (protons) and electrons, wherein the electrons flow from anode 16 through an electrically connected external load 21, and the protons migrate through a membrane 22 to cathode 20. At cathode 20, the protons and electrons react with the oxygen gas to form resultant water 14xe2x80x2, which additionally includes any reactant water 14 dragged through membrane 22 to cathode 20. The electrical potential across anode 16 and cathode 20 can be exploited to power an external load.
The same configuration as is depicted in FIG. 1 for a fuel cell is conventionally employed for electrolysis cells. In a typical anode feed water electrolysis cell (not shown), process water is fed into a cell on the side of the oxygen electrode (in an electrolysis cell, the anode) to form oxygen gas, electrons, and protons. The electrolytic reaction is facilitated by the positive terminal of a power source electrically connected to the anode and the negative terminal of the power source connected to a hydrogen electrode (in an electrolysis cell, the cathode). The oxygen gas and a portion of the process water exit the cell, while protons and water migrate across the proton exchange membrane to the cathode where hydrogen gas is formed. In a cathode feed electrolysis cell (not shown), process water is fed on the hydrogen electrode, and a portion of the water migrates from the cathode across the membrane to the anode where protons and oxygen gas are formed. A portion of the process water exits the cell at the cathode side without passing through the membrane. The protons migrate across the membrane to the cathode where hydrogen gas is formed.
In certain arrangements, the electrochemical cells can be employed to both convert electricity into hydrogen, and hydrogen back into electricity as needed. Such systems are commonly referred to as regenerative fuel cell systems.
The typical electrochemical cell includes a number of individual cells arranged in a stack, with the working fluid directed through the cells via input and output conduits formed within the stack structure. The cells within the stack are sequentially arranged, each including a cathode, a proton exchange membrane, and an anode. The anode, cathode, or both are conventionally gas diffusion electrodes that facilitate gas diffusion to the membrane. Each cathode/membrane/anode assembly (hereinafter xe2x80x9cmembrane electrode assemblyxe2x80x9d, or xe2x80x9cMEAxe2x80x9d) is typically supported on both sides by support members such as screen packs or bipolar plates, forming flow fields. Since a differential pressure often exists in the cells, compression pads or other compression means are often employed to maintain uniform compression in the cell active area, i.e., the electrodes, thereby maintaining intimate contact between flow fields and cell electrodes over long time periods.
In addition to providing mechanical support for the MEA, flow fields such as screen packs and bipolar plates preferably facilitate fluid movement and membrane hydration. These porous support members can also serve as gas diffusion media to effectuate proper transport of the oxygen and hydrogen gas. Increasing the rates of transport and uniformity of distribution of the fluids (including liquid water and oxygen and hydrogen gases) throughout the electrochemical cells increases operating efficiencies.
With the support of the screen packs, conventional electrochemical cells can operate at a pressure differential of up to about 400 pounds per square inch (psi). While suitable for their intended purposes, such support members require additional manufacturing materials and steps, and may not be effective for cells operating at differential pressures greater than about 400 psi. In an electrolysis cell, for example, it is desirable to operate the cell at about 1,000 psi or greater. At pressures exceeding about 400 psi, and especially exceeding 600 psi, physical limitations of screen structures, i.e., the requirement of very small screen openings, hinders fluid transport therethrough, and thus limits their usefulness.
In order to enable operation at pressures up to about 2,000 psi, porous plate technology has accordingly been developed. An exemplary porous plate is disclosed in U.S. Pat. Nos. 5,296,109 and 5,372,689, issued to Carlson et al. in 1994. As shown in FIG. 2, a porous sheet 213 is disposed between the anode electrode 211 and the flow field (screen pack 203) to provide additional structural integrity to the membrane 209. According to Carlson, porous sheet 213 further enables dual-directional water and oxygen flow.
Porous plate have also been previously disclosed in a paper presented at The Case Western Symposium on xe2x80x9cMembranes and Ionic and Electronic Conducting Polymerxe2x80x9d, Cleveland, Ohio May 17-19, 1982. Again as illustrated in FIG. 2, this paper discloses that in order to prevent the membrane and electrode assembly from deforming into the flow fields, a porous, rigid support sheet 213 is inserted between the electrode 211 and the flow field distribution component 203. The particular arrangement described employs a porous titanium support sheet on the anode electrode, and a carbon fiber paper, porous, rigid support sheet on the cathode electrode (p. 14). At page 2, this paper claims that such cells were capable of operating at differential pressures ranging up to greater than 500 psi.
Use of porous plates are also disclosed in xe2x80x9cSolid Polymer Electrolyte Water Electrolysis Technology Development for Large-Scale Hydrogen Productionxe2x80x9d, Final Report for the Period October 1977-November 1981 by General Electric Company, NTIS Order Number DE82010876, which is directed to solid polymer electrolyte water electrolysis technology. Certain electrolyzer arrangements using porous titanium plates are described, and as shown in FIG. 2 include an anode electrode 211, an anode electrode flow field of molded grooves 203, a cathode electrode 207, a cathode flow field of molded grooves 205, and ion exchange membrane 209 disposed between and in intimate contact with anode 211 and cathode 207. A porous sheet 213 is shown supporting ion exchange membrane 209 and interposed between anode flow field 203 and anode electrode 211. It is stated on page 10 of the report that earlier development had shown that a support for the membrane and electrode assembly was required on both the anode and cathode side to prevent creep of the membrane into flow areas. The anode support (porous sheet 213) comprised a thin, titanium foil with many small holes etched through for transport of water to the catalyst from the flow field and outflow of the oxygen gas. To provide improved water flow rates, these thin foil anode supports were reported to be replaced with porous titanium plates (p. 66).
A significant disadvantage of porous plate technology is the additional materials, manufacturing, and assembly expense that this element adds to the cell assembly. What is accordingly needed in the art is a cost effective electrochemical cell capable of operating at high pressures, e.g. exceeding about 1,000 psi.
The above-described drawbacks and disadvantages are alleviated by an electrochemical cell comprising a porous electrode, another electrode, and a membrane disposed between said electrodes; the method for using the electrochemical cell; the methods for making a porous electrode; and the method for producing electrical power.
The electrochemical cell comprises: a first, porous electrode, a second electrode, and a membrane disposed therebetween, wherein said first, porous electrode comprises a catalyst disposed in physical contact with an electrically conductive, porous support; a flow field in fluid communication with said second electrode; a first fluid port in fluid communication with said first electrode; and a second fluid port in fluid communication with said second electrode. The first, porous electrode accordingly comprises a porous catalytic structure which provides structural support for and integrity to the catalyst, reactive sites for the electrolysis of water to hydrogen and oxygen, a fluid flow field for the working gases and fluids, and support for the membrane.
One method for using the electrochemical cell comprises: introducing water to an oxygen electrode, wherein said oxygen electrode comprises a catalyst disposed in physical contact with an electrically conductive porous electrode; dissociating the water to form hydrogen ions, oxygen, and electrons; moving said electrons through an external load to a hydrogen electrode; migrating said hydrogen ions through a electrolyte membrane to the hydrogen electrode; and producing hydrogen gas at said hydrogen electrode.
One method for making the porous electrode comprises: sintering a layer of electrically conductive material to form a sintered, porous support; imbibing said sintered porous support with a solution of catalyst and solvent; and removing the solvent to form the porous electrode, wherein said porous electrode preferably has a porosity greater than about 20% by volume.
An alternative method for making the porous electrode comprises: coating an electrically conductive material with a solution of catalyst and solvent; forming a layer of said coated electrically conductive material; optionally removing the solvent from said layer; and sintering said layer to form the porous electrode, wherein said porous electrode preferably has a porosity greater than about 20% by volume.
Still another method for making the porous electrode comprises: coating an electrically conductive, porous support with a solution of catalyst precursor and solvent; and converting said catalyst precursor to a catalyst, wherein said porous electrode has a porosity preferably greater than about 20% by volume.
The method for producing electrical power comprises: producing first electricity; introducing at least a portion of said first electricity to an electrochemical cell having an oxygen electrode, a hydrogen electrode, an electrolyte membrane disposed therebetween, and an electrical load in electrical communication with said oxygen electrode and said hydrogen electrode, said oxygen electrode comprising a catalyst disposed in an electrically conductive porous flow field; introducing water to said oxygen electrode; dissociating the water to form hydrogen ions, electrons, and oxygen; migrating said hydrogen ions through said electrolyte membrane to said hydrogen electrode; moving said electrons through said electrical load to said hydrogen electrode; producing hydrogen gas at said hydrogen electrode; and using said hydrogen gas to produce additional electricity when said first electricity is not available or is insufficient.
The above discussed and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description and drawings.